The present invention is in the field of aminotransferase proteins that are related to the aspartate aminotransferase subfamily, recombinant DNA molecules, and protein production. The present invention specifically provides novel peptides and proteins that effect protein phosphorylation and nucleic acid molecules encoding such peptide and protein molecules, all of which are useful in the development of human therapeutics and diagnostic compositions and methods.
Aminotransferases
Aminotransferases are enzymes that catalyze the transfer of amino groups from .alpha.-amino to .alpha.-keto acids. They are also called transaminases. The alpha.-amino groups of the 20 L-amino acids commonly found in proteins are removed during the oxidative degradation of the amino acids. The removal of the .alpha.-amino groups, the first step in the catabolism of most of the L-amino acids, is promoted by aminotransferases (or transaminases). In these transamination reactions, the .alpha.-amino group is transferred to the .alpha.-carbon atom of .alpha.-ketoglutarate, leaving behind the corresponding .alpha.-keto acid analog of the amino acid. There is no net deamination (i.e., loss of amino groups) in such reactions because the .alpha.-ketoglutarate becomes aminated as the .alpha.-amino acid is deaminated. The effect of transamination reactions is to collect the amino groups from many different amino acids in the form of only one, namely, L-glutamate. The glutamate channels amino groups either into biosynthetic pathways or into a final sequence of reactions by which nitrogenous waste products are formed and then excreted.
Cells contain several different aminotransferases, many specific for .alpha.-ketoglutarate as the amino group acceptor. The aminotransferases differ in their specificity for the other substrate, the L-amino acid that donates the amino group, and are named for the amino group donor. The reactions catalyzed by the aminotransferases are freely reversible, having an equilibrium constant of about 1.0 (.DELTA.G.sup.0xe2x80x2.congruent.0 kJ/mol).
Aminotransferases are classic examples of enzymes catalyzing bimolecular ping-pong reactions. In such reactions the first substrate must leave the active site before the second substrate can bind. Thus the incoming amino acid binds to the active site, donates its amino group to pyridoxal phosphate, and departs in the form of an .alpha.-keto acid. Then the incoming .alpha.-keto acid is bound, accepts the amino group from pyridoxamine phosphate, and departs in the form of an amino acid.
The measurement of alanine aminotransferase and aspartate aminotransferase levels in blood serum is an important diagnostic procedure in medicine, used as an indicator of damage to the heart and other organs and to monitor recovery from the damage. For example, measurement of aspartate aminotransferase isoenzymes is used to determine the extent of liver necrosis and for determining prognosis in hepatic disease, as well as for diagnosing active alcoholic liver disease. Measurement of aspartate aminotransferase isoenzymes in acute myocardial infarction provides additional diagnostic information not provided by other tests used in the art, such as creatine kinase and lactate dehydrogenase-based tests (Panteghini, Clin. Biochem. 23 (4), 311-319 (1990)).
Several heart and liver diseases have been correlated with abnormally high levels of serum aspartate transaminase (AST). Examples of such conditions include acute myocardial infarction, pulmonary emulsion, acute pancreatitis, viral and toxic hepatitis, and acute cirrhosis. Generally speaking, AST is elevated in diseases affecting tissues rich in AST.
Extensive studies have shown that 92-98% of patients with acute myocardial infarction have elevated serum AST level. The measured levels are usually four to ten times the upper limit of normal values. The elevated AST levels develop six to twelve hours after the time of infarction and usually return to normal by the third or fourth day. Secondary rises can be correlated with other features, suggesting extension or recurrence of myocardial infarction. Also, mild elevations of serum AST levels have been reported in patients with pulmonary infarction. In patients with congestive heart failure and those with marked tachycardia, mild to moderate degrees of AST elevation may occur. These have been attributed to hepatic necrosis secondary to hepatic congestion. Patients with pericarditis also have been reported to have a fifty percent incidence of slightly elevated AST levels.
Striking elevations in AST levels are observed in the serum of almost all patients with acute hepatic necrosis. In patients with cirrhosis of the liver there is a 60-70% incidence of elevated AST levels. Obviously the early detection of an abnormal rise in AST levels can lead to more rapid and accurate diagnosis of heart and liver disease.
Elevated AST levels have even been correlated with various cancers. Approximately half the patients with metastatic carcinoma have elevated serum AST levels in the same range as patients with cirrhosis and posthepatic jaundice. Less frequently such moderately elevated AST levels are observed in patients with lymphoma and leukeumia. See, Todd-Sanford, Clinical Diagnosis By Laboratory Methods, W. B. Saunders Co., 14th Ed., pp. 693-723 (1969).
Accordingly, the identification of a new member of the aminotransferase family of proteins, particularly one related to the aspartate aminotransferase, provide targets for examining protein turnover in response to a pathological or biological process.
The present invention is based in part on the identification of amino acid sequences of human aminotransferase peptides and proteins that are related to the aspartate aminotransferase subfamily, as well as allelic variants and other mammalian orthologs thereof. These unique peptide sequences, and nucleic acid sequences that encode these peptides, can be used as models for the development of human therapeutic targets, aid in the identification of therapeutic proteins, and serve as targets for the development of human therapeutic agents that modulate aminotransferase activity in cells and tissues that express the aminotransferase. Experimental data as provided in FIG. 1 indicates expression in humans in the testis.